I have a new 1Tb WDWE II. I ran the rebuilding to RAID1 21 hours ago and the drive still doesn't respond. I did everything according to the manual. The drive haven't been hacked yet. Now it just ticks quietly (like moving the harddrive heads), the fan spins, blue leds are constantly on, and I can't power it off with the power button. Network access doesn't respond. I'm afraid that if I plug the power off I'll kill it.
So how long should it actually take, from your experience?
Update: 32 hours, still running :)

I have a 6 disk x 200GB RAID 5 array in a different Linux box. It's far faster, with a lot more memory, possibly better IDE I/O, etc.

By coincidence, I had to replace one of the failed 200GB disks last night. That rebuild took around 3 hours.

Therefore, if it took 3 hours to write 200GB of data, it would seem logical that it would take a minimum of 15 hours to write 1TB of data. And that's on much faster hardware. So I think your 21 hours might not be unreasonable.

That being said, most any other system would still respond, albeit slowly. I'm sorry if that's not much help, but I thought I'd try.